Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
307 PM EDT Wed Aug 22 2018

Valid Aug 22/1200 UTC thru Aug 26/0000 UTC

...See NOUS42 KWNO (ADMNFD) for the status of the upper air
ingest...

12Z Model Evaluation...with Preferences and Confidence
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...Overall Synoptic Pattern across the CONUS...
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Preference: General model blend; but with less weight given to the
NAM across the Ohio Valley Fri-Fri night.

Confidence: Average

The 12z runs of the NAM, GFS, ECMWF, UKMET, and Canadian Global
are in generally good agreement into Fri with a 700-500 mb wave
crossing the northern Plains into the upper MS Valley and a trough
extending south and downstream that crosses out of the central
Plains and MS valley into the Ohio Valley. Layer Fri is when the
NAM gets out of synch with its faster movement of the wave across
the Ohio Valley, continuing into the lower Great Lakes and upper
OH Valley Sat. There is decent clustering among the
GFS/ECMWF/UKMET/Canadian global, so a blend of these models is
recommended.

Further west -- the models are in pretty good agreement with the
trough moving into the northwestern U.S. late Fri.  The NAM builds
more of a downstream short amplitude 500 mb ridge into central and
then eastern MT Fri-Fri evening with a lower amplitude 500 mb
trough in WA Sat afternoon, with better agreement among the
ECMWF/GFS/UKMET/12z GEFS Mean on retaining a 500 mb trough in WA.

Model trends at www.wpc.ncep.noaa.gov/html/model2.shtml
500 mb forecasts at www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml

Petersen